Subterranean deposits of coal, also referred to as coal seams, contain substantial quantities of entrained resources, such as coal seam gas (including methane gas or other naturally occurring gases). Production and use of coal seam gas from coal deposits has occurred for many years. However, substantial obstacles have frustrated more extensive development and use of coal seam gas deposits in coal beds.
In the past, coal seam gas was extracted through multiple vertical wells drilled from the surface into the subterranean deposit. Coal seams may extend over large areas of up to several thousand acres. Vertical wells drilled into the coal deposits for obtaining methane gas can drain only a fairly small radius into the coal deposits around the wells. Therefore, to effectively drain a coal seam gas deposit, many vertical well bores must be drilled. Many times, the cost to drill the many vertical well bores is not justified by the value of the gas that is expected to be recovered.
Horizontal drilling patterns have been tried in order to extend the amount of coal seam exposed to a drill bore for gas extraction. However, horizontal drilling patterns require complex and expensive drilling equipment, for example, for tracking location of the drilling bit and directionally drilling drainage patterns. Consequently, drilling horizontal patterns is expensive and the cost must be justified by the value of the gas that will be recovered.
In many instances it is necessary to enlarge a well bore that has a tubing residing therein. The manner in which underreamer tools for enlarging a well bore are conventional configured does not allow the underreamer tool to be used in a well bore that has a tubing.